only throw IOExc if merge really hit an exception

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1595240 13f79535-47bb-0310-9956-ffa450edef68
Merged revision(s) 1595247 from lucene/dev/trunk:
More cleanup, sorry for the bug I introduced! :(

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1595252 13f79535-47bb-0310-9956-ffa450edef68